When you decide to travel by air, you will produce emissions. Roughly ninety percent of emissions are generated by the planes, two percent comes from the airport, and eight percent from getting to and from the airport.
Obviously, it is almost impossible for us to reduce emissions from the plane or at the airport. However, the remaining eight percent, get to and from the airport, is up to your decision. This is the best means of transportation that you should consider arranged from best to worst.
Definitely the best option because you are sharing the ride with dozens of others. In addition, public transit system normally uses cleaner fuel such as compressed natural gas and low-sulfur diesel. Public transit may take a little longer, but it will cost less that other options.
Use a shared-ride van
Many airports are served by shared-ride vans and usually they are more environmentally friendly than driving yourself because they carry more people per gallon of gas burned.
It seems counter-intuitive, but this is a better option than having someone else drive you. This is because you are eliminating at least two unnecessary trips. If someone takes you to the airport, s/he needs to drive back afterwards. Then if someone picks you up when you get back, the person has to make a trip to the airport.
Hail a cab
This has similar emissions as drive yourself option because taxis normally pick people up for both legs of an airport run. But if you are not stay in a big city, the taxi driver may need to drive extra miles after drop you off at your home before finding another passanger.
Ask someone to drive you
This is the worst option because of the two unnecessary trips explained above.
The complete transcript of this strategy is available in How to cut your emissions when traveling by air.